This invention relates to methods and apparatus for automatically re-igniting a vacuum arc plasma source, in particular a direct current (DC) vacuum arc or a long pulse AC vacuum arc, and in particular to such methods and apparatus that are capable of enhancing vacuum arc deposition by minimizing pressure fluctuations in the deposition process.
Vacuum metal arc deposition offers high deposition rates and produces dense coatings by the use of dense, energetic plasmas of electrically conducting elements. The high ion flux is created from minute, extremely hot, and discrete locations on the arc cathode known as cathode spots. In order to ignite the arc, a mechanical metal trigger can be used, but the ignition process may generate contamination that should preferably be avoided especially in semiconductor applications. Frequently, the arc plasma extinguishes during the deposition process, and in dual plasma deposition involving both gaseous and metallic elements, the chamber pressure can surge significantly within a few seconds after the arc extinction because a large amount of gas is bled in to react with the metallic ions in the cathodic plasma to produce the film. It is in practice very difficult to immediately adjust the gas flow after the are extinction, and the sudden burst of gas may result in inhomogeneity in the composition and structure of the film. Thus, rapid re-ignition that can minimize this problem is desirable in many applications.
Apparatus for automatic re-ignition of a vacuum arc, comprising: (a) means for sensing a main power supply voltage, (b) means for generating an ignition signal, and (c) control means for controlling said ignition signal generating means in response to an output from said main power supply voltage sensing means whereby an ignition signal is generated when said main supply voltage is above a predetermined value.
Preferably the ignition signal generating means comprises a pulse generating means, wherein pulses generated by the generating means are used to control switch means for transferring stored energy from an energy storage means to a spark component, and wherein the control means controls the application of the pulses to the switch means.
In a first embodiment of the invention the pulse generating means generates a continuous series of pulses and the control means allows the pulses to be applied to the switch means only when the main power supply voltage is above the predetermined value.
In a second embodiment of the invention the control means causes the pulse generating means to generate pulses only when the main power supply voltage is above the predetermined value.
Preferably the switch means comprises a thyristor.
Since different cathodes and different plasma arcs may extinguish at different points, preferably the main power supply sensing voltage includes means for adjusting the predetermined voltage to a desired value. Preferably, the predetermined voltage is fVw where Vw is the normal working voltage when the arc is established, and Vi/Vw less than f less than 1 where Vi is the idle voltage of the main power supply when the arc is extinguished. The main power supply voltage sensing means may comprise first and second resistors in series, and wherein f is determined by the values of the first and second resistors. One of the two resistors may be a variable resistor to provide the control of the value of the main power supply voltage considered to be indicative of arc extinction.
According to another broad aspect of the present invention there is provided a method for automatic re-ignition of a vacuum arc, comprising sensing a main power supply voltage, and generating an ignition signal when said main power supply voltage is above a predetermined value indicative of the arc becoming extinguished.